This invention relates to an optical transmission system, an optical transmission line and an optical transmitter, and more specifically to an optical transmission system, an optical transmission line and an optical transmitter applicable for long haul and large capacity transmission.
In recent years, there has been a big demand for data transmission and accordingly long haul and large capacity transmission lines have been desired. In order to realize the long haul and large capacity transmission, it is necessary to appropriately control nonlinear effect and chromatic dispersion as described by M. Murakami et al. in xe2x80x9cLong-Haul 16xc3x9710 WDM Transmission Experiment Using Higher Order Fiber Dispersion Management Technique,xe2x80x9d ECOC""98, 20-24 September, Madrid, Spain and Japanese Disclosure Gazette No. 10-221562 (U.S. Pat. No. 5,781,673).
The paper of Murakami et al. discloses a configuration in which two optical fibers, having chromatic dispersion values of opposite signs and approximately equal lengths, are disposed in one repeater span, and chromatic dispersion values and dispersion slopes of the two kinds of the optical fibers are selected so that accumulated chromatic dispersions become zero at a certain target wavelength (a central wavelength 1550.7 nm of a signal wavelength band) and difference between the accumulated chromatic dispersions within the signal wavelength band is reduced. Furthermore, it describes that, in each repeater span, the optical fiber disposed in front should have a lager mode-field diameter of 9.2 xcexcm compared to 5.7 xcexcm of the optical fiber disposed in behind so as to reduce the non-linear effects.
Also, the Japanese Patent Disclosure Gazette No. Heisei 10-221562 discloses a configuration in which the following elements are disposed in series; a first optical fiber having non-zero chromatic dispersion within a signal wavelength band, a second optical fiber having chromatic dispersion with a mathematical sign opposite to that of the first optical fiber, and a third fiber for compensating dispersion slope in the signal wavelength band. It is also described in the Gazette that the second optical fiber makes the total chromatic dispersion of the whole transmission line to be zero with a certain wavelength in the signal wavelength band, and the third fiber makes the dispersion slope to be xe2x88x920.1 ps/nm2/km.
Concretely, experimental results of 34 wavelengthsxc3x9710 Gbit/s WDM transoceanic optical transmission systems such as explained below have been reported (e.g. K. Matsuda et al. xe2x80x9c340 Gbit/s (34xc3x9710 Gbit/s) WDM transmission over 8,514 km using broadband gain equalization technique for transoceanic systems,xe2x80x9d Electronics Letter, Vol. 35, pp. 1090-1091, 1999).
If such dispersion control described in the above paper is used, it is practically impossible to realize a terabit class WDM optical transmission system that enables long haul transoceanic optical transmission over 7000 km by multiplexing 100 wavelengths of 10 Gbit/s or so. It is therefore necessary to develop more advanced systems.
An object of the present invention is to provide an optical transmission system, an optical transmission line and an optical transmitter for realizing even larger capacity and longer haul transmission.
An optical transmission system according to the invention comprises an optical transmitter for outputting WDM signal light, an optical transmission line having a plurality of first optical amplification repeater spans and transmitting the WDM signal light output from the optical transmitter, and an optical receiver for receiving the WDM signal light propagated on the optical transmission line, wherein the first optical amplification repeater span of the optical transmission line comprises a first optical repeater amplifier for optically amplifying the WDM signal light, an optical transmission fiber of plus chromatic dispersion for transmitting the WDM signal light output from the optical repeater amplifier, and a local area dispersion compensator of minus chromatic dispersion for compensating accumulated chromatic dispersion caused by the optical transmission fiber so that average chromatic dispersion in the span becomes a predetermined value Dlocal as well as compensating a dispersion slope to become practically zero, wherein Dlocal is no less than 1 ps/nm/km and no more than 4 ps/nm/km.
An optical transmission line according to the invention comprises a plurality of optical amplification repeater spans and transmits WDM signal light, wherein each optical amplification repeater span comprises a first optical repeater amplifier for optically amplifying the WDM signal light, an optical transmission fiber of positive chromatic dispersion for transmitting the WDM signal light output from the optical repeater amplifier, and a local area dispersion compensator of minus chromatic dispersion for compensating accumulated chromatic dispersion caused by the optical transmission fiber so that the average chromatic dispersion in the span becomes a predetermined value Dlocal as well as compensating a dispersion slope to become practically zero, wherein Dlocal is no less than 1 ps/nm/km and no more than 4 ps/nm/km.
With this configuration, an optical transmission line with a little nonlinear effect and flat chromatic dispersion characteristics can be obtained, and it also becomes possible to narrow signal wavelength intervals. Consequently, dense wavelength multiplexing can be realized, and accordingly the long haul and high capacity transmission can be realized in combination with appropriate control of the chromatic dispersion.
Preferably, the optical transmission system further comprises a second optical amplification repeater span disposed at every wide area dispersion compensation cycle composed of a plurality of the first optical amplification repeater spans, the second optical amplification repeater span comprises a second optical repeater amplifier for optically amplifying the WDM signal light and having gain smaller than that of the first optical repeater amplifier, and a wide area dispersion compensator for compensating the dispersion slope to become practically zero as well as compensating the accumulated chromatic dispersion of the WDM signal light so that average chromatic dispersion in the wide area dispersion compensation cycle becomes a predetermined value Davg. With this configuration, the frequency that the accumulated chromatic dispersion passes across the zero point is reduced and therefore the accumulated chromatic dispersion of the whole optical transmission line can be controlled to keep a low value.
Preferably, the optical transmission line according to the invention further comprises a second optical repeater amplifier for optically amplifying the WDM signal lights output after propagating the plurality of the optical amplification repeater spans, the second optical repeater amplifier having gain smaller than that of the first optical repeater amplifier, and a wide area dispersion compensator for compensating accumulated chromatic dispersion of the WDM signal lights output from the second optical repeater amplifier so that the average chromatic dispersion becomes a minus predetermined value Davg as well as compensating a dispersion slope to become practically zero. With this configuration, the frequency that the accumulated chromatic dispersion passes across the zero point is reduced and therefore the accumulated chromatic dispersion of the whole optical transmission line can be controlled to keep a low value.
Preferably, in both inventions, the second optical repeater amplifier comprises an optical amplifier having the same gain with the first repeater amplifier and an attenuator for attenuating output light from the optical amplifier to become a predetermined level. With this configuration, it becomes possible to equalize loss of every repeater span and hence the optical repeater amplifiers with the same gain characteristics can be used, making gain profile control and maintenance control to be simplified.
Preferably, Dave should be set practically from xe2x88x920.3 to xe2x88x920.1 ps/nm/km. This reduces the frequency that the accumulated chromatic dispersion passes across the zero point and hence spectrum spreading can be decreased. Accordingly, high capacity and long haul transmission such as 1 Tbit/s transmission of 8000 km can be realized.
Preferably, an effective core area should be set to 110 xcexcm2 or more. This drastically reduces the nonlinear effect.
Preferably, a mode field conversion optical system should be disposed between the optical transmission fiber and the local area dispersion compensator. With this configuration, the optical transmission fiber and the local area dispersion compensator having largely different mode field diameters can be optically connected with low loss.
In the optical transmission system according to the invention, the optical transmitter preferably comprises a plurality of signal light generators for outputting signal lights respectively having a wavelength different from each other and a polarization combiner for combining the signal lights output from the signal light generators so that polarizations of adjacent wavelength channels become orthogonal. Since the polarization directions of the adjacent channels are different directions, channel-to-channel interaction such as XPM (Cross Phase Modulation) can be reduced even when the intervals of the signal wavelengths are narrowed.
In the optical transmission system according to the invention, the optical transmitter preferably comprises a plurality of signal light generators for outputting signal lights respectively having a wavelength different from each other, a first wavelength multiplexing element for wavelength-multiplexing the signal lights with even-numbered wavelengths output from the signal light generators in the same polarization, a second wavelength multiplexing element for wavelength-multiplexing the signal lights with odd-numbered wavelengths output from the signal light generators in the same polarization, and a polarization combiner for combining output lights from the first and second wavelength multiplexing elements in mutually orthogonal polarizations from each other. With this configuration, the adjacent wavelength channels can be combined in the different directions of polarization using only a few polarization-combining elements.
In the optical transmission system according to the invention, the optical transmitter preferably comprises a plurality of signal light generators for outputting signal lights respectively having a wavelength different from each other, the plurality of signal light generators being divided into a plurality of groups in order of wavelength, a plurality of wavelength multiplexers for wavelength-multiplexing the signal lights output from the respective groups of the signal light generators, a plurality of optical filters for removing unnecessary band components from the respective output lights of the plurality of the wavelength multiplexers, a combiner for combining output lights from the plurality of the optical filters, and a plurality of chromatic dispersion imparters for imparting predetermined amounts of chromatic dispersion to the signal lights of the respective groups. With this configuration, S/N ratio of the signal light being output onto the optical transmission line can be improved and as a result the transmission distance can be extended.
In the optical transmission system according to the invention, the optical transmitter preferably comprises a plurality of signal light generators for outputting signal lights respectively having a wavelength different from each other, the signal light generators being divided into first and second groups, a first wavelength multiplexing element for wavelength-multiplexing the signal lights with even-numbered wavelengths output from the first group of the signal light generators in the same polarization, a second wavelength multiplexing element for wavelength-multiplexing the signal light with odd-numbered wavelengths output from the first group of the signal light generators in the same polarization, a first polarization combiner for combining the output lights from the first and second wavelength multiplexing elements in mutually orthogonal polarizations, a first optical filter for removing unnecessary band components from output light of the first polarization combiner, a third wavelength multiplexing element for wavelength-multiplexing the signal lights with the even-numbered wavelengths output from the second group of the signal light generators in the same polarization, a fourth wavelength multiplexing element for wavelength-multiplexing the signal lights with the odd-numbered wavelengths output from the second group of the signal light generators in the same polarization, a second polarization combiner for combining output lights from the third and fourth wavelength multiplexing elements in mutually orthogonal polarizations, a second optical filter for removing unnecessary band components from output light of the second polarization combiner, a combiner for combining the output lights from the first and second optical filters, and first and second chromatic dispersion imparters for imparting predetermined amounts of chromatic dispersion to the signal lights in the first and second groups respectively. With this configuration, when extremely many signal lights such as 100 wavelengths are wavelength-multiplexed, it can be performed efficiently since adjacent wavelength channels have different polarization directions.
In the optical transmission system according to the invention, preferably, the first chromatic dispersion imparter is connected to an output of the combiner, and the second chromatic dispersion imparter is disposed between the second polarization combiner and the combiner. With this configuration, the first chromatic dispersion imparter imparts a predetermined chromatic dispersion to all signal lights and the second chromatic dispersion imparter imparts a necessary chromatic dispersion to the signal lights in the second group. The total length of the chromatic dispersion imparter can be shortened compared to the case that chromatic dispersion is individually imparted to the respective groups.
The optical transmitter according to the invention comprises a plurality of signal light generators for outputting signal lights respectively having a wavelength different from each other, the plurality of signal light generators being divided into a plurality of groups in order of wavelength, a plurality of wavelength multiplexers for wavelength-multiplexing the signal lights output from respective groups of the signal light generators, a plurality of optical filters for removing unnecessary band components from the respective output lights of the plurality of the wavelength multiplexers, a combiner for combining output lights from the plurality of the optical filters, and a plurality of chromatic dispersion imparters for imparting predetermined amounts of chromatic dispersion to the signal lights of respective the groups. With this configuration, S/N ratio of the signal light being output onto the optical transmission line can be improved and as a result the transmission distance can be extended.
The optical transmitter according to the invention comprises a plurality of signal light generators for outputting signal lights respectively having a wavelength different from each other, the plurality of signal light generators being divided into first and second groups, a first wavelength multiplexing element for wavelength-multiplexing the signal lights with even-numbered wavelengths output from the first group of the signal light generators in the same polarization, a second wavelength multiplexing element for wavelength-multiplexing the signal lights with odd-numbered wavelengths output from the first group of the signal light generators in the same polarization, a first polarization combiner for combining the output lights of the first and second wavelength multiplexing elements in mutually orthogonal polarizations, a first optical filter for removing unnecessary band components from output light of the first polarization combiner, a third wavelength multiplexing element for wavelength-multiplexing either signal lights with the even-numbered wavelengths output from the second group of the signal light generators in the same polarization, a fourth wavelength multiplexing element for wavelength-multiplexing the other signal lights with the odd-numbered wavelengths output from the second group of the signal light generators in the same polarization, a second polarization combiner for combining the output lights of the third and fourth wavelength multiplexing elements in mutually orthogonal polarizations, a second optical filter for removing unnecessary band components from output light of the second polarization combiner, a combiner for combining the output lights from the first and second optical filters, and first and second chromatic dispersion imparters for imparting predetermined amounts of chromatic dispersion to the signal lights in the first and second groups respectively.
With this configuration, when extremely many signal lights such as 100 wavelengths are wavelength-multiplexed, it can be performed efficiently since adjacent wavelength channels have different polarization directions.
Preferably, the first chromatic dispersion imparter is connected to an output of the combiner, and the second chromatic dispersion imparter is disposed between the second polarization combiner and the combiner. With this configuration, the first chromatic dispersion imparter imparts a predetermined chromatic dispersion to all signal lights and the second chromatic dispersion imparter imparts a necessary chromatic dispersion to the signal lights in the second group. The total length of the chromatic dispersion imparter can be shortened compared to the case that chromatic dispersion is individually imparted to the respective groups.
The optical transmission system according to the invention comprises an optical transmitter for outputting signal light, an optical transmission line for transmitting the signal light output from the optical transmitter, and an optical receiver for receiving the signal light transmitted on the optical transmission line, wherein the optical transmission line comprises a plurality of optical transmission fibers for transmitting the signal light, a plurality of optical repeater amplifiers for optically amplifying the signal light, one or more wide area dispersion compensators, disposed every wide area dispersion compensation cycle having a plurality of optical repeater spans determined by the optical repeater amplifiers, for compensating the chromatic dispersion of the signal light so that the average chromatic dispersion value of the whole transmission line becomes a value equal to a minus predetermined value Davg, wherein Davg is no less than xe2x88x920.3 ps/nm/km and no more than xe2x88x920.1 ps/nm/km, and a plurality of local area dispersion compensators, each disposed after the optical transmission fiber in each predetermined optical repeater span within the wide area dispersion compensation cycle, for compensating chromatic dispersion of the signal light output from the optical transmission fiber so that the average chromatic dispersion value in each optical repeater span becomes a predetermined value Dlocal, wherein Dlocal is no less than 1 ps/nm/km and no more than 4 ps/nm/km.
Also, the optical transmission line according to the invention comprises a plurality of optical transmission fibers for transmitting signal light, a plurality of optical repeater amplifiers for optically amplifying the signal light, one or more wide area dispersion compensators, disposed every wide area dispersion compensation cycle having a plurality of optical repeater spans determined by the optical repeater amplifiers, for compensating chromatic dispersion of the signal light so that the average chromatic dispersion of the whole transmission line becomes to a value equal to a minus predetermined value Davg, wherein Davg is no less than xe2x88x920.3 ps/nm/km and no more than xe2x88x920.1 ps/nm/km, and a plurality of local area dispersion compensators, each disposed after the optical transmission fiber in a predetermined optical repeater span within the wide area dispersion compensation cycle, for compensating chromatic dispersion of the signal light output from the optical transmission fiber so that the average chromatic dispersion in each optical repeater span becomes a predetermined value Dlocal, wherein Dlocal is no less than 1 ps/nm/km and no more than 4 ps/nm/km.
With these configurations, the nonlinear effect and the chromatic dispersion can be highly balanced and hence the long haul transmission characteristics are improved. Since the frequency that the accumulated chromatic dispersion passes across the zero point is reduced, deterioration of signal spectrum can be reduced. This also leads to the improvement of the transmission characteristics.
Preferably, in the optical repeater span in which the wide area dispersion compensator is disposed, an attenuator is disposed in front of the wide area dispersion compensator, the attenuator has a predetermined loss amount for adjusting loss in the optical repeater span to become a predetermined value. With this configuration, it becomes possible to equalize losses of all repeater spans and hereby the optical repeater amplifiers with the same gain characteristics can be used making file control and maintenance control easier.
Preferably, the wide area dispersion compensator compensates a dispersion slope of the signal light to become practically zero, and the local area dispersion compensator compensates the dispersion slope of the signal light to become practically zero. With this configuration, an optical transmission line with a little nonlinear effect and flat chromatic dispersion characteristics can be realized, and also signal wavelength intervals can be narrowed. Consequently, dense wavelength multiplexing is realized, and thus long haul and large capacity transmission can be realized combined with proper control of the chromatic dispersion.
An effective core area of the optical transmission line is preferably no less than 110 xcexcm2. With this value, the nonlinear effect can be greatly reduced.
Preferably, a mode field conversion optical system is disposed between the optical transmission fiber and the local area dispersion compensator. With this configuration, the optical transmission fiber and the local area dispersion compensator, both having quite different mode field diameters, can be connected at low loss.